In the case of a conventional vehicle suspension for a road vehicle which is required to move over uneven surfaces, there are two suspension features which are fundamentally undesirable, the first being that the spring must be capable of supporting much more than the weight of the vehicle, and if therefore a wheel is to negotiate a rise in the surface over which the vehicle travels, it is required to deflect the spring with a force much greater than that proportion of the vehicle weight which is applied to the wheel, and secondly, at the same time it is required to displace liquid in a shock absorber which, under extreme conditions, can apply very high forces to the chassis of the vehicle. Even further difficulties are encountered when a vehicle arranged to carry heavy loads is relatively lightly loaded, in that the ratio of spring deflection over applied force is so small that the result is a "rough ride". This has been compensated for in the past almost entirely by the mass of the sprung weight, or high inertia structure, and the wheel base dimensions. Thus a large motor car for example will give a much smoother ride than a small motor car. Furthermore, a lightly loaded vehicle may have reduced adhesion between the wheels and the road (if the road surface is rough). This results in some loss of control of the vehicle.
Improvements have been introduced in some suspension structures, and for example in one suspension structure which is well known, gas is used as the spring medium, and a variable quantity of hydraulic oil is interposed to maintain the body at constant height for variations in load. However even with this arrangement recovery of a vehicle to normal height after a variation of load takes several seconds to complete, and the system does not compensate for certain dynamic changes, for example nose dip under braking or squat under acceleration.
The need for automatic levelling suspension means for vehicles has been well recognised, and for example the Australian Pat. No. 226,525 issued to Thompson Products Inc. and the Austrailan Pat. No. 406,399 issued to Monroe Auto Equipment Co. both relate to devices for levelling of a vehicle, but in neither instance is provision made for reducing dynamic "rough ride" forces, responding to dynamic forces such as produced by road surface variations and reducing their effect on the vehicle.
There has also been considerable attention paid to the need to control body attitude, and for example in Australian Pat. No. 428,824 issued to Automotive Products Company Limited there is described and claimed a device utilising a pivotally suspended weight to effect such control, but again there is no provision for reducing "rough ride" forces, responding to dynamic forces such as produced by road surface variations and reducing their effect on the vehicle.